The present invention relates to a storage subsystem in which data is stored. More particularly, the present invention relates to the cooperation control of a plurality of storage devices that form the storage subsystem in which duplicated data is stored. “A Case for Redundant Arrays of Inexpensive Disks (RAID)” (Proceedings of ACM SIGMOD, 1988) stipulates the classification of disk array devices, composed of a plurality of disk devices, according to the generation of redundant data and the storage method.
In the above technology, a disk array device is called RAID1 that stores duplicated data on two disk devices and that allows data to be read from or write to one of the disk devices even when an error occurs on the other disk device in the redundant configuration and data cannot be read from or written to that disk device.
In addition, JP-A-10-74129 (first prior art) discloses a technology that reduces the load on the write data transfer from the disk controller to the duplicated disk devices in the RAID1 disk array device described above to improve the processing performance of the disk controller.
According to the first prior art, the disk controller transfers write data only to the first disk device when writing data and the first disk device, in turn, transfers the write data to the second disk device not via the disk controller, eliminating the need for transferring write data from the disk controller to the second disk device.
In addition, JP-A-7-281959 (second prior art) discloses a method for connecting the two disk devices via a dedicated communication line to allow them to write data at the same time while communicating one another during the data write operation.
According to the second prior art, write data is transferred to the two disk devices at the same time. Therefore, the amount of write data on the interface is the same as that of write data when data is not duplicated.
The first prior art described above at least requires a disk controller that controls the disk devices of the disk array device and that has the means for issuing a special write command to the first disk device. This makes the disk array device expensive.
In addition, the prior art described above does not reduce the amount of write data transferred on the interface, that is, does not reduce the load on the interface between the disk controller and the disk devices. Therefore, the prior art does not prevent performance degradation caused by a conflict on the interface in the disk array device.
The second prior art eliminates the need for the disk controller that is the problem with the first prior art and prevents an increase in the amount of write data transferred on the interface. However, implementing this technology requires a dedicated communication line through which the disk devices communicate.
In addition, the two disk devices synchronize one another, or the master device monitors the operation progress of the slave device, to transfer write data. Therefore, a decrease in the disk device usage efficiency or an extra time required for writing data on one disk device degrades performance.
Furthermore, in the prior art described above, much attention is not paid to the method for switching between the first device and the second device or between the master device and the slave device, or to the disk device selection control method during data read processing.